I recently bought a Canon rebel XT (I live in Holland and here it's called eos 350D) and I wanted to know what filter holder I can buy. I want to use (square) ND grad filters. Can anyone give me some general info or links on the topic?

The filter company Cokin makes a wide range of holders for square filters and the filters themselves. Just Google the name and you'll get their website.
Lots of photography retailers handle their stuff, including B&amp;H Photo.
Other names in the square filter biz include Hitech, Kodak, Lee and Optiflex, but Cokin may be the most widely available.
Grant

Yes Cokin makes a wide variety of filters. I'm still using the set I bought for my 35mm camera over 20 years ago. One of the nice things about the Cokin filters is that when you switch cameras all you need to change is the adapter ring. All your filters will stile work.

The polarizer filter is always useful. If you take pictures of candles or night shots you might want to consider the star filter. If you do landscapes the graduated blue filter comes in handy.

There are so many filters availible I'm sure you'll find anything you'd need.